Ultrasonic reflection-based transient temperature gradient inversion for compensating oil film thickness measurement

The propagation nature of sonic waves in a temperature gradient field is often simplified with uniform temperature assumptions, resulting in compensation errors in ultrasonic-based oil film measurement. Focusing on this, a novel compensation method of inverting the temperature gradient via ultrasonic signal is proposed. For thrust bearings, the time shift of the reflected signal from the substrate-coating interface is adopted as the inversion index for the heat source. Numerically, the temperature gradient can be derived by solving the heat conduction equation using the finite difference method, then the ultrasound time shift can be theoretically obtained by substituting the acoustic characteristics into each temperature node. In practice, with the measured time shift of the reflection signal, the temperature gradients can be inverted after iterative solutions of the theoretical model. The measurement effectiveness of temperature and oil film thickness have been experimentally validated using a thrust bearing rig, where the ultrasound measurement results were compared with data from thermometric sensors and eddy current sensors.